1. Field of the Invention
The present invention is directed to a method and an apparatus for forming a superconducting layer onto the large substrate surface. The present invention is, in particular, suitable for depositing a superconducting layer which includes complex superconducting oxides.
2. Related Background Art
Laser ablation is one of the most suitable deposition process for obtaining excellent quality of thin films on substrate. Laser ablation process has advantages, such as excellent stoichiometric stability of deposited film, when the processes are used for the films of the superconducting oxide compounds. Laser ablation also has another advantage that it does not require any electromagnetic fields, which may possibly affect the charged particles contained in the deposited film. Therefore, laser ablation is expected to be employed as deposition process for most promising near-term devices.
Complex oxide superconducting compounds such as Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x are the multi-elements complex oxide materials. The stoichiometric stability is very important for forming films of the complex oxide superconducting compounds, since a slight stoichiometric deviation causes a serious deterioration of the superconduction characteristics of the formed films. As mentioned above, laser ablation is suitable for depositing thin films of the complex oxide superconducting compounds with excellent stoichiometric stability. Thus, studies have been made to obtain excellent complex oxide superconducting films having excellent superconduction characteristics. For example, Japanese Patent No. 53818/04 (Kokoku Hei 04-53818) discloses a processing method for forming a Y-Ba-Cu-O complex oxide superconducting film on a copper sheet, by irradiating CO.sub.2 laser beam onto a cylindrical shaped sintered Y-Ba-Cu-O target, under the condition of the total pressure of 1.times.10.sup.-4 -1 Torr and the O.sub.2 partial pressure of 5.times.10.sup.-5 -1 Torr.
Linzen et al. (Second International Conference on Macroscopic Quantum Phenomena, Smolenice Castle, Czechoslovakia, Aug. 31 Sep. 4, 1992) discloses experiments of forming Y-Ba-Cu-O superconducting films via laser ablation using KrF excimer laser under the condition of various oxygen (O.sub.2) partial pressures which varied from 10 Pa (about 75 mTorr) to 150 Pa (about 1.13 Torr). Linzen et al. further discloses a study concerning the influence of the oxygen partial pressure upon the surface condition and the superconduction characteristics of the formed Y-Ba-Cu-O films. Krebe et al. (J. Appl. Phys. 69(4), pp.2405-2409, February 1991) discloses experiments of forming Y-Ba-Cu-O superconducting films via laser ablation under the condition of the oxygen partial pressure ranging from 0.008 mbar (about 6 mTorr) to 2 mbar (1.5 Torr). Either Linzen et al. and Krebs et al. points out that the surface condition and the superconduction characteristics of the resultant Y-Ba-Cu-O films become worse when the oxygen partial pressure is in excess of 1 Torr. Another reference (Reference No. OQD-92-53, "Shadanhojin Denki Gakkai, Hirari Ryosi Debaisu Kenkyukai Shiryo (Japan Electrical Society, Photon and Quantum Devices Study meeting Proceedings)", pp.69-77, Oct. 28, 1992.) discloses a method for forming Y-Ba-Cu-O superconducting films with high quality via laser ablation using excimer laser.
Other studies concerning laser ablation have been also made by, for example, Song et al. (Appl. Phys. Lett. 63(24), pp.3370-3372, December 1993), Foltyn et al. (Appl. Phys. Lett 59(11), pp.1374-1376, September 1991) and J. A. Greer and H. J. VanHook (SPIE Proceedings, vol. 137, November 1990). In particular, Greer et al. discloses a method for producing thin films of a variety of oxide compounds over large areas.
The deposition process by laser ablation is typically carried out in the process chamber which is capable of being highly evacuated, and any gas can be introduced into the process chamber, as described in the above reference. In the operation via laser ablation, a substrate and a target are placed in the chamber, and laser beam is introduced into the process chamber from the external laser beam source to strike the target surface. The heated target material sublimes, and is deposited onto the substrate surface.
The deposited substrate via conventional laser ablation generally includes undesired particles on the deposited surface. Thus, it is difficult to achieve sufficient smoothness of the deposited surface of the substrate by the conventional laser ablation. Since the smoothness of the film surface contributes to the uniformity of performances and characteristics of the deposited films, it is difficult to form sufficient superconducting film having uniform characteristics via conventional laser ablation method.
The off-axis laser deposition method has recently been proposed, for decreasing undesired particles on the deposited films. For example, Holzapfel et al. discloses an off-axis laser deposition of YBa.sub.2 Cu.sub.3 O.sub.7-x thin films (Appl. Phys. Lett., vol. 61, No. 26, pp. 3178-3180, Dec. 28, 1995).
Holzapfel et al. carried out their off-axis laser ablation deposition under the chamber pressure of about 0.4 mbar (about 0.3 Torr). However, the quality of the film deposited under such a chamber pressure was not generally acceptable for superconducting elements.
Another problem of the laser ablation is that the available deposition surface area is considerably small, so that forming superconducting material films onto large substrate is difficult.
Therefore, it is desirable to provide a laser ablation method which presents an excellent film quality.
It is also desirable to provide a laser ablation method which is used for forming films onto the large substrate with a uniform film quality.